mt_monashmedfandomcom-20200213-history
L0301P71 - Haematopoietic Stem Cells and Differentiation
__TOC__ Components of Haemopoiesis Analogy *seeds = haematopoietic stem cells (HPC) *soil = bone marrow niche *fertilised + H2O = growth factors Composition of Blood *centrifuging blood forces formed elements to separate from the plasma *plasma = 55% of whole blood *buffy coat: leukocytes and platelets = <1% of whole blood *erythrocytes = 45% of whole blood *∴ hematocrits - percentage of total volume that is cells = ~45% Blood Cells *formed daily: red = 2x1011 white = 1x1010 *constant replenishment of mature blood cells by proliferation and “commitment” of stem cells to the various lineages Hierarchy of Stem Cells *most primitive HSC **capable of proliferation but remains quiescent until needed *multipotent progenitors (MPP) **forms myeloid and lymphoid precursors *common myeloid precursor (CMP) gives rise to: **megakaryocyte-erythroid progenitor ***gives rise to megakaryocytes and erythrocytes *granulocyte-macrophage progenitor **gives rise to monoblasts (to be monocytes) and myeloblasts (to be granulocytes) *common lymphoid precursor (CLP) gives rise to B, T, NK cells **bipotent progenitors have been identified: ***B lymphoid/macrophage and T lymphoid/myeloid Assays for Stem Cells *in vivo assays identify stem cells with different characteristics: **longterm repopulating cells (LTRC) ***capable of producing all blood cell types for the entire life span **short repopulating cells (STRC) ***reconstitute myeloid and/or lymphoid compartments for a short period of time Recognition of Stem Cells *flow cytometry - based on proteins expressed on the surface of the cells Negative Selection *as stem cells should not express commitment marker such as CD2( (T cells), CD19 (b cells), CD14 (monocytes), CD15 (granulocytes), CD16 (NK cells), glycophorin (erythroid) Positive Selection *as most stem cells are CD34+ Other Discriminants *low uptake of rhodamine-123 *forward and side scatter characteristics *sedimentation rate Haemopoiesis in the Embryo *1 month: yolk sac and Aorta-Gonad-Mesonephros (AGM) **contains multipotent HSC **megakaryocytes delayed until week 5 **lymphocytes until week 6 *3 months: foetal liver and spleen *5 months: first bone marrow cavities form **initially site of granulopoiesis and megakaryopoiesis **at 6 months, major site of erythropoiesis transfers to marrow space *foetal erythropoiesis is entirely megaloblastic until 11 weeks, and partially until day 5 postnatally Importance of Self Renewal and Differentiation *self renewal - maintains the stem cell pool *differentiation - creates specialised cells that replace dead or damaged cells throughout your life Self Renewal of HSC Two Models #fixed number of stem cells created in embryogenesis *successively and randomly recruited into proliferation into lineage progenitors and differentiation into mature cells #small number of stem cells at embryogenesis *capable of self renewal to generate cells of same totipotency and a proportion undergo commitment and maturation Differentiation of HSC *stochastic **random commitment to a particular lineage *instructive **determination by cell-cell contact or by the effect of diffusible signals such as growth factors Transcriptional Control of Haemopoiesis *transcription factors (TFs) **proteins that direct cells into a specific roles by differential gene expression *selective activation of a TF determines lineage commitment *without particular TFs, some cells may never be formed Examples *PU.1 - master myeloid switch **highest expression in myeloid cells and also in B cells but not T cells *GATA-1 - erythroid development regulator Growth Factor: Colony Stimulating Factors *multiplication and differentiation of HSC is under the influence of colony stimulating factors (CSF) *may be relatively specific for a cell lineage (erythropoietin), or may influence multiple lineages (GM-CSF) *bind to specific receptors on the surface of cells and initiate a signalling cascade *are used in the accelerating recovery of blood cells of chemotherapy Growth Factors Involved in Haemopoiesis Obtaining Stem Cells *originally requires taking a few litres of been marrow form the pelvis - but very painful and dangerous *now, amplification of stem cell numbers by: **induce mobilisation: releasing them from the bone marrow niche using G-CSF ***releases neutrophil elastase which free HSCs into the blood stream *differentiation of stem cells into committed progenitors and terminally differentiated mature cells Use of Growth Factors in Chemotherapy *chemotherapy drugs are nonselective *low blood counts are an important side effect and precent administration of full-dose treatment *thus blood cell transplants are used: **collect and store stem cells **administrate the high dose chemotherapy **reinject the stem cells and provide growth factors to stimulate differentiation Haemopoiesis Postnatal Changes At birth: *only in the bone marrow >4 years: *increasing numbers of fat cells in marrow 25 years: *only active sites are skull bones, ribs and sternum, scapulae and clavicles, vertebrae, pelvis and upper half sacrum, proximal femur and humerus *even at active sites, 50% of marrow contains fat In disease states: *reversion to extramedullary sites of HP (spleen, liver - swelling occurs) *increase ratio of haemopoietic to fat cells *extension of haemopoietic cells into cavities normally containing only fat Cellular Components of Bone Marrow *vascular endothelial cells *stromal **reticular fibroblasts **adipocytes **osteoblasts **osteoclasts *haemopoietic cells **erythroblasts **megakaryocytes **lymphocytes **macrophages **granulocytes Leukocyte Production (Leukopoiesis) *committed cell types:  B & T progenitors and granulocyte-macrophage colony-forming units **possess receptors for CSFs **released by mature WBCs in response to infections *bone marrow stores and releases granulocytes and monocytes *some lymphocytes leave bone marrow to complete their maturation **e.g. T cells - go to thymus *circulating WBCs do not stay in bloodstream **granulocytes leave in 8 hours and live 5 days longer **monocytes leave in 20 hours, become macrophages and live for several years **WBCs providing long-term immunity last decades Granulopoiesis *forming granulocytes **include eosinophils and basophils *regulated by a combination of cytokines **includes GM-CSF, IL-3, stem cell factor Platelet Production (Thrombopoiesis) *stem cell that develops receptorsfor thrombopoietin becomes megakaryoblast *megakaryoblast repeated replicates its DNA without dividing **forms gigantic cell called megakaryocyte *in foldings of megakaryocytic cytoplasm splits off cell fragments that enter the bloodstream as platelets (live for 4 days) *some stored in spleen and can be released as needed *regulated by thrombopoietin (TPO), stem cell factor, IL-6  Monocytopoiesis *regulated by M-CSF, IL-3 and GM-CSF Erythrocytes (RBCs) *disc-shaped cell with thick rim **7.5 μM diameter & 2.0 μm thick at rim *live for 120 days **membrane fragility —> lysis in narrow channels in the spleen, capillaries Sickle Cell Anaemia *RBC are no deformable thus no sufficient oxygen transport around the body = pain Gas Transport *major function of erythrocytes is to transport gases around the body **lost all organelles during maturation so has increased surface area/volume ratio ***increases diffusion rate of substances in and out of cell **33% of cytoplasm is haemoglobin (Hb) ***O2 delivery to tissue and CO2 transport back to lungs **contains enzyme carbonic anhydrase ***produces carbonic acid from CO2 and water which plays an important role in gas transport and pH balance Erythropoiesis *produces 2.5 million RBCs/second *first committed cell is proerythroblast **has receptors for EPO from kidneys *erythroblasts multiply and synthesise Hb *nucleus degenerates to form a reticulocyte **named for fine network of ER **enters bloodstream as 0.5 to 1.5% of circulating RBCs *development takes 3-5 days, involves **reduced cell size, increased cell number, synthesis of Hb and loss of nucleus Erythropoietin (EPO) *produced by kidney *increased in anaemia, hypoxic states, renal tumours *decreased in renal disease and in primary polycythaemia *stimulates erythropoiesis up to 8-fold over basal levels Homeostasis *classic negative feedback control **drop in RBC count causes hypoxemia (low O2 in blood) to kidneys **EPO production ↑ **stimulation of bone marrow **RBC count ↑ in 3-4 days *stimulus for erythropoiesis **low levels of atmospheric O2 **increase in exercise **haemorrhaging